The present invention relates, in general, to mixers used in wireless communications systems and, more specifically, to a mixer that reduces the effect of the harmonics of the local oscillator signal, particularly the second harmonic.
A xe2x80x9cGilbert Mixerxe2x80x9d is widely used in radio frequency communications systems. FIG. 1 is a circuit diagram of a conventional xe2x80x9cGilbert Mixer.xe2x80x9d Mixer 10 includes a mixer core 13, which includes transistors Q1, Q2, Q3, and Q4, and a radio frequency input section 15, which includes transistors Q5 and Q6.
The operation of mixer 10 is as follows. In the absence of any voltage difference between the base of transistor Q5 and the base of transistor Q6, the collector currents of these two transistors are essentially equal. Thus, a voltage applied to the local oscillator port 12 results in no change of output current. Should a small DC offset voltage be present at the radio frequency port 14 (e.g., due to a mismatch in the emitter areas of transistors Q5 and Q6), this will only result in a small feed through of the local oscillator signal VLO to the intermediate frequency output port 16 which will be blocked by a first intermediate frequency filter (not shown). Conversely, if a radio frequency signal VRF is applied to the radio frequency port 14, but no voltage difference is applied to the local oscillator port 12, the output currents will again be balanced. A small offset voltage (due now to emitter mismatch in transistors Q1, Q2, Q3, and Q4) may cause some radio frequency signal feed through to the intermediate frequency port 16. As before, this will be rejected by the intermediate frequency filter. Thus, it is only when a signal supplied to both the local oscillator port 12 and the radio frequency port 14 that a signal appears at the intermediate frequency port 16. To reduce the gain and increase the input linearity of the mixer 10, a capacitor 18 and a resistor 20 are connected between the base and the collector of transistor Q5 and a capacitor 22 and a resistor 24 are connected between the base and the collector of transistor Q6.
Of concern with such a mixer is the possibility of high second harmonic contents in the emitters of the quad transistors Q1, Q2, Q3, and Q4 when the input level of the local oscillator signal is high. The presence of the second harmonic, which is of most concern because it has the greatest effect due to its magnitude when compared to the other harmonics, can degrade the conversion gain and linearity of the mixer.
One object of the present invention is to provide a new and improved mixer for a wireless communications system.
Another object of the present invention is to provide a new and improved mixer for a wireless communications system that compensates for harmonics of the local oscillator signal, particularly the second harmonic.
A further object of the present invention is to provide a mixer for a wireless communications system that is relatively simple in construction and inexpensive to manufacture.
Accordingly, a mixer, constructed in accordance with the present invention, includes means for supplying a first input signal and means for supplying a second input signal. This mixer also includes an input section circuit responsive to the first input signal for translating the first input signal from a voltage signal to first and second current signals. The input section circuit includes (a) first and second transducers connected in a differential configuration and each having an output for supplying one of the first and second current signals, (b) a first reactance circuit connected between a first of the transducer outputs and ground, and (c) a second reactance circuit connected between a second of the transducer outputs and ground. A mixer, constructed in accordance with the present invention, further includes a mixer core circuit responsive to the first and second current signals and the second input signal for developing a differential mixed output signal that is the product of the current signals and the second input signal.
It is to be understood that the foregoing general description of the present invention and the following detailed description of the present invention are exemplary, but are not restrictive of the invention.